Steffen Suchandt

Automatic Extraction of Traffic Flows Using TerraSAR-X Along-Track Interferometry

Spaceborne synthetic aperture radar (SAR) offers great potential for the measurement of ground traffic flows. A SAR with multiple receiving apertures aligned in flight direction repeatedly images the same ground area with a short time lag. This allows for an effective detection of moving ground objects, whose range variation translates into an interferometric phase signal between the receiving channels. The high-resolution German SAR satellite TerraSAR-X offers several ways to create multiple along-track apertures. We exploit this to demonstrate satellite-based traffic-flow measurements using along-track interferometry (ATI) and Displaced Phase Center Array techniques. In this paper, we address the usage of different TerraSAR-X ATI modes for data acquisition and describe an automatic near-real-time processing chain for the extraction of traffic information. The performance of this TerraSAR-X traffic processor is significantly driven by incorporating a priori knowledge of road networks. We present examples of automatic traffic detection as well as empirical evaluations thereof using different kind of reference data.

Current Measurements in Rivers by Spaceborne Along-Track InSAR

The global monitoring of river discharges is a technologically challenging problem, with important applications in a variety of disciplines. Due to the limited availability and/or quality of river runoff data from many regions, an increasing use of remote sensing techniques is highly desirable. Altimeter specialists have already demonstrated water level retrievals in rivers from available data. The along-track interferometric synthetic aperture radar [along-track InSAR (ATI)] capabilities of state-of-the-art imaging radars on satellites such as the German TerraSAR-X, which was launched on June 15, 2007, also permit high-resolution line-of-sight surface current measurements. In this paper, we evaluate the potential of current measurements in rivers by spaceborne ATI on the basis of fundamental theoretical considerations, existing spaceborne InSAR data from the shuttle radar topography mission (SRTM), and simulated TerraSAR-X data. We show that an SRTM-derived line-of-sight surface current field in the Elbe river, Germany, agrees well with numerical hydrodynamic model results. The data quality is sufficient to resolve characteristic lateral variations of the currents in the river around a pronounced main flow channel. Assuming that the flow direction is usually aligned with the river bed, even a quasi-2D total surface current field can be derived. Simulations indicate that TerraSAR-X was even better suited for current measurements in rivers. Depending on width, surface roughness, and relative flow direction of a river, current estimates with an accuracy better than 0.1 m/s were possible with an effective spatial resolution of a few hundred meters to kilometers.

Quality Assessment of Surface Current Fields From TerraSAR-X and TanDEM-X Along-Track Interferometry and Doppler Centroid Analysis

All existing examples of current measurements by spaceborne synthetic aperture radar (SAR) along-track (AT) interferometry (ATI) have suffered from short baselines and corresponding low sensitivities. Theoretically, the best data quality at X-band is expected at effective baselines on the order of 30 m, i.e., 30 times as long as the baselines of the divided-antenna modes of TerraSAR-X. In early 2012, we had a first opportunity to obtain data at near-optimum baselines from the TanDEM-X satellite formation. In this paper, we analyze two TanDEM-X interferograms acquired over the Pentland Firth (Scotland) with effective AT baselines of 25 and 40 m. For comparison, we consider a TerraSAR-X dual-receive-antenna (DRA)-mode interferogram with an effective baseline of 1.15 m, as well as velocity fields obtained by Doppler centroid analysis (DCA) of single-antenna data from the same three scenes. We show that currents derived from the TanDEM-X interferograms have a residual noise level of 0.1 m/s at an effective resolution of about 33 m × 33 m, while DRA-mode data must be averaged over 1000 m × 1000 m to reach the same level of accuracy. A comparison with reference currents from a 1-km resolution numerical tide computation system shows good agreement in all three cases. The DCA-based currents are found to be less accurate than the ATI-based ones but close to short-baseline ATI results in quality. We conclude that DCA is a considerable alternative to divided-antenna mode ATI, while our TanDEM-X results demonstrate the true potential of the ATI technique at near-optimum baselines.

SRTM X-SAR calibration results

The paper presents the results of the Shuttle Radar Topography Mission (SRTM) X-band calibration at DLR as of April 2001, the time of writing. While some areas may be subject to further improvement, the overall image will not change significantly. We summarize the various calibration issues, the methods applied and the results obtained. Addressed topics are: Timing calibration, SNR and coherence, motion analysis and instrument phase errors. The qualitative and quantitative effects of the distortions above on the DEM quality are discussed.

Direct Surface Current Field Imaging from Space by Along-Track InSAR and Conventional SAR

Since the SEASAT mission in 1978, spaceborne synthetic aperture radar (SAR) images of ocean scenes have been used successfully for a variety of applications. However, despite the fact that a SAR is a Doppler radar, conventional SAR images do not provide direct information on target velocities, since all Doppler information is normally utilised to obtain the best possible spatial resolution under the assumption that targets are not moving. Recent technological progress has enabled us to overcome this shortcoming to some extent: The split-antenna mode of TerraSAR-X permits an acquisition of two SAR images with a short time lag for along-track interferometry, and advanced processing of conventional SAR raw data, demonstrated with ENVISAT ASAR data, permits velocity retrievals at a reduced spatial resolution even with a single antenna. We describe the methodologies and the theoretical advantages and limitations of both approaches, review experimental results from the last decade, and discuss possible and desirable future developments.

Detection and velocity estimation of moving vehicles in high-resolution spaceborne synthetic aperture radar data

Automatic estimation of traffic parameters has evolved to an important topic of research. Current and upcoming SAR satellite missions offer new possibilities for traffic monitoring and control from space as an alternative to conventional traffic data acquisition. In this paper a detection approach is presented which evaluates simultaneously the effects moving objects suffer from in the SAR focusing process. Information about the measured signal and the expected signal are utilized in the detection framework. Analyses of the proposed technique are done with real spaceborne SAR data.

Comparison of DEMs derived from SRTM/X- and C-band

In February 2000, the Shuttle Radar Topography Mission (SRTM) successfully mapped the entire landmass between 60deg N and 54deg S using an Interferometric Synthetic Aperture Radar (InSAR). The data were acquired in Cand X-band and were independently processed to Digital Elevation Models (DEM) by NASA-JPL (C-band) and DLR (X-band). A commonly used source was the Position and Attitude Determination Record (PADR) generated by the Attitude Orbit Determination Avionics (AODA) system. The PADR file provides the orbit and baseline information. All C- and X-band data were systematically processed to 1 arc-second resolution DEMs. The X-band DEMs are globally available at DLRs German Remote Sensing Data Center (DFD). USGS provides the C-band derived elevation data in 1 arc-second for the US and globally a reduced version with 3 arc-second spacing. The presentation compares the X- and C-band derived elevation models. Starting from the individual verification results the correspondence of the SRTM DEMs with respect to height and location accuracy is investigated. Potential global and local discrepancies are discussed. Both datasets are compared to the Altimetry Corrected Elevations global DEM (ACE)

Recovering radar shadow to improve interferometric phase unwrapping and DEM reconstruction

This letter presents a new approach to actively use radar shadow in order to unwrap and geocode objects that cannot be reconstructed by synthetic aperture radar interferometry alone. This new algorithm may significantly improve phase unwrapping and digital elevation model reconstruction in mountainous topography or in urban areas. The algorithm is explained and examples are given with data from the Shuttle Radar Topography Mission, where shadow is a serious problem in Alpine terrain.

Results from an airborne SAR GMTI experiment supporting TerraSAR-X traffic processor development

The launch of the advanced high resolution radar satellite TerraSAR-X in summer 2006 opens new possibilities for the demonstration of traffic monitoring from space. DLR is currently developing an operational traffic processor for the TerraSAR-X ground segment. The paper presents results from an airborne SAR GMTI campaign that was part of a study for algorithm development and processor design. DLR’s E-SAR sensor was used in an Along-Track Interferometry (ATI) mode to image vehicles in controlled and uncontrolled situations. The paper gives an overview on the experiment and presents the results of across- and along-track velocity estimation. Adapted SAR processing techniques were applied to enhance the peak energy of the moving objects in the focused SAR images. The paper presents first results and discusses the techniques.

Experiences with SRTM/X-SAR phase unwrapping using the minimum cost flow method

With the Shuttle Radar Topography Mission (SRTM) flown in February 2000, the most comprehensive interferometric SAR data of the Earths land mass so far were acquired. At the German Aerospace Center the X-SAR data are processed to a precise and homogeneous Digital Elevation Model (DEM). One of the most critical processing steps is phase unwrapping (PU). We are using an implementation of the Minimum Cost Flow (MCF) algorithm. Now that a total of 16,000 interferograms between /spl plusmn/60/spl deg/ latitude have been processed, this offers the unique opportunity to study the behavior of the MCF algorithm using a very large data pool. In this paper we report on the practical experiences with MCF gained during operational SRTM/X-SAR data processing. We present the results of a throughput analysis. Further, the problems experienced with MCF PU of the X-SAR data are discussed. It is shown how specific adoptions of the MCF cost functions can help to overcome them. The paper also presents the methods of PU quality assessment used in our operational processing.